Ended up going with 4mm sheet (going in to the base, and possibly in the rear standard to reduce some weight). I wanted to go with 3mm; however, getting a decent 3mm router bit proved too difficult.

CF is reasonably easy to work with. Lighter weight sheeting can be cut with a decent blade (Titanium edge is good). I'll be cutting this with a jigsaw, and then throwing away the blade. You need to cover the cut with masking tape to stop tearing, and it's a good idea to wet the sheet and/or wear a mask as well. Once cut, you finish it off with 400 Grit sandpaper.